1. Field of the Invention
This invention embodies a novel series of low viscosity liquid crystal mixtures wherein the components include one or more binary mixtures of two short chain ester liquid crystals of essentially equal molecular length. Liquid crystal (LC) mixtures have been used in many electro-optical (E-O) devices, in lieu of single LC compounds, in order to widen the operating temperature range of these devices and, in some instances, to improve their electro-optical characteristics.
Certain E-O devices which operate in a dynamic scattering (DS) mode, such as LC reticles and LC matrix displays, require liquid crystal mixtures which exhibit low viscosities in addition to other desirable characteristics for E-O devices such as a wide nematic range, good dopant solubilities, high conductivity anisotropy, negative dielectric anisotropy, good birefringence, colorlessness, and stability to light, heat, moisture and electrical signals. The availability of LC mixtures exhibiting appropriate combinations of these characteristics is particularly limited.
2. Description of the Prior Art
Prior art LC materials used in electro-optical devices are commonly mixtures of LC components, combined to achieve appropriate anisotropic properties and the required nematic temperature range. Often multi-component mixtures are used, and trade-off factors are involved in trying to optimize the various properties obtained from the LCs which are combined. Previously, LC mixtures were used in which all of the LC components differed in molecular length, usually by two or more methylene groups and rarely by as little as one methylene group in the total number of alkyl and alkoxyl end group carbon atoms. This sharply limited the combinations that could be used to prepare short length mixtures with wide temperature ranges and other desirable properties such as low viscosity, good dopant solubility as well as favorable anisotropic properties for device applications. LC components are chosen whose mixtures have depressed melting points (crystalline to nematic phase changes) with a minimum that generally corresponds to a eutectic mixture composition. Nematic components which strongly interact to form molecular complexes result in mixtures with a complexity of phases, and such mixtures were generally avoided for E-O devices. On the other hand, components which are too much alike can form mixtures which behave as solid solutions. These are generally not useful because the melting point range obtainable is no lower than the lowest of the pure components.
Previously, the most common method of achieving eutectic mixtures has been to combine different molecular length LC compounds from the same homologous series, i.e. combined compounds whose difference is the sum of the length of their terminal R and R' end groups. This approach has been used to obtain eutectic mixtures from many different series of LC compounds. For example, binary mixtures of 4,4'-di-n-alkoxyazoxybenzenes were studied and it was found that eutectic mixtures were obtained if "the ratio of the molecular length is equal or less than ca.0.80." (D. Demus, C. Fietkau, R. Schubert and H. Kehlen, Mol. Cryst. Liq. Cryst., B 25, 215-232 (1974). These compounds have the following structure: ##STR1## Where R is an alkyl group having from 1 to 6 carbon atoms. It can be concluded from the Demus et al studies that with C.sub.n representing the number of carbons in each of the two end groups, that the binary mixtures of C.sub.2 /C.sub.3, C.sub.3 /C.sub.4, C.sub.4 /C.sub.5 and C.sub.5 /C.sub.6 did not show good eutectic properties, while good properties were obtained from C.sub.1 /C.sub.2, C.sub.1 /C.sub.3, C.sub.1 /C.sub.4, C.sub.1 /C.sub.5, C.sub.1 /C.sub.6, C.sub.2 /C.sub.4, C.sub.2 /C.sub.5, C.sub.2 /C.sub.6, C.sub.3 /C.sub.6, and C.sub.4 /C.sub.6. The C.sub.1 /C.sub.2 case was noted to be the only exception to the 0.8 ratio-of-lengths rule. However, even the C.sub.1 and C.sub.2 components differed by two carbons in length, while the other mixtures differed by 4 or more carbons in length.
When azoxy LC compounds are synthesized with alkoxy (RO) and alkyl (R') end groups, isomeric mixtures are generally obtained. These are used in mixtures in which the sum of the R and R' groups differ by two carbon atoms such as in the commercially available Merck Nematic Phase 5 mixture which consists of the following two compounds and their isomers: ##STR2##
The general practice of using different length nematic compounds for mixtures also pertains to other LC classes such as Schiff bases and esters. For example, the nematic temperature range of Schiff base mixtures are reported, in U.S. Pat. No. 3,540,796 issued to J. E. Goldmacher et al on Nov. 17, 1970, to be widened by using mixtures such as the following where the R plus R' total is different by three carbons: ##STR3## However, the following mixture, where R plus R' is only one carbon different showed only a very slight decrease in its melting point (m.p.) (45.degree.) compared to the m.p. of its lower melting component (49.degree.): ##STR4## A one carbon difference has a larger effect when it is the different between a methyl and ethyl group such as in the well known MBBA/EBBA mixture ##STR5## which does have a depressed m.p. compared to its two components.
Many ester mixtures with widened temperature ranges have been prepared from different molecular length mixtures from the same homologous series. For example, the commercially available Merck Nematic Phase 1052 Licristal uses the following binary mixture in which R plus R' differs by five carbons: ##STR6## Similarly, an acyloxy mixture (Merck Nematic Phase 1008 Licristal) is a binary mixture in which R plus R' differs by four carbon atoms ##STR7##
Different classes of esters, not all part of a homologous series, have also been combined to form wide temperature range nematic mixtures, but generally the sums of the R plus R' end groups are different, or else an entirely different end group is used in place of one of the alkyl or alkoxy groups. Thus, there is a distinct difference in molecular length of the components, and often in the end group structure as well. One example of this is the HRL-2N10 mixture, which contained one dialkyl phenyl benzoate and three dialkoxy phenyl benzoates, that was reported by H. S. Lim et al, in Appl. Phys. Lett., 28, 478 (1976). All of the components in this mixture are of distinctively different molecular lengths.
Another example of a wide nematic range ester mixture with combined classes which have different end groups as well as different molecular lengths is shown in U.S. Pat. No. 4,000,084 issued to P. Y. Hsieh, et al on Dec. 28, 1976.
All of these examples are shown to explain and illustrate the prior art of combined LC components to widen the nematic temperature range of mixtures. Similar components with essentially the same molecular length (for example, constant values for the R+R' carbon and phenyl benzoate) have been avoided, presumably to avoid solid solution formation. We found that even a mixture of the esters p-hexyloxyphenyl p'-butylbenzoate (60-4) and p-hexyloxyphenyl p'-pentylbenzoate (60-5) which differ in length by only one methylene group, forms a solid solution phase diagram.